I make mine the same way Bethe does: a square of aluminum form a soda can ( you can cut it easily with scissors ) a sewing needle, and emery paper. I use 600 grit paper. I spin the needle with light pressure until it just pokes through, then sand on both sides a lot, then repeat. The hole is always way too small to begin with. It gets bigger as I keep working on it and with some practice you'll get an eye for when it's about right.

Yeah. The needle isn't for pushing through the foil to make a hole, it's for making a bump in the foil, and then you sand the bump off to make a hole. There are several advantages to doing it that way, including: i) the size of the hole is easier to control; ii) the hole is nice and round; iii) it has clean edges; iv) the edges of the hole are thin (this is important if you're making a hole in a thicker kind of foil, like for example a piece of beercan).

I usually use a knife sharpening stone to grind the bump down, because it's very fine grit, it's flat and it's rigid so it doesn't deform while I'm trying to make a nice precise round hole.

I tried today a smaller hole. 0.390mm that is smaller the the ideal 0.422 but flare become an issue I think it is because that.

And the image came less sharp than the 0.500mm hole. I don't know if it is because the pinhole is too small or if it is because I loaded three paper in the camera hich moved the exposing paper fowards. Or could it be both?

I just went back through this thread and I can't find the "focal" length of your camera. But I'd think .39 would be about perfect if optimal is 0.42 for film. Flare can be a problem ( or a feature!! ) any time the sun hits the pinhole directly. I do not think the distance the paper moved from having 3 papers loaded could matter. Maybe there was some light leak around the papers after all?

I ignore reciprocity failure in paper negatives, but have a suspicion it is there in very long exposures. The few times I have made exposures over 10 minutes, the effect has not been as strong as I expected, and I've noticed some strange things that might be different speeds in the different parts of the VC emulsion when the exposure gets over about 10 minutes.

I too would like to know if other people think about reciprocity failure with paper. Or if anyone has noticed this "color separation effect" between greens and blues in long VC paper exposures.

You might check with your dentist. There are round dental burs that are 0.4mm. Many dental labs have drill press instruments that will hold the bur and make an accurate cut. My original understanding of your thread related to calculating exposure times, if that is a concern the Ilford exposure calculator I described has given me excellent results.

Using Pinhole Designer with a user constant of 1.4 instead of the default 1.9 and a light wavelength of 500nm, the suggested diameter for a focal length of 100mm is .313mm. The optimum diameter has been argued for well over a hundred years, and isn't that critical for most photography. For macro pinhole photography, the diameter should be reduced even further http://huecandela.com/hue-x/pin-pdf/...%20Wellman.pdf.

I haven't noticed any reciprocity failure in 10 minute enlargements, but any failure might be masked by rather casual evaluation of prints.

As for color separation in pinhole photography, it might be present, although masked by the normal unsharpness of pinhole images. Pinholes do have very slight chromatic aberration.

My technique for making pinholes is similar to edp's. A very hard smooth stone is ideal for the smaller pinholes. For the smallest ones, I keep an ordinary sewing pin in the hole while stoning down the dimple. This gives more control over the pinhole diameter and leaves less of a burr than stoning the dimple without the pin. A sheet of .002" brass shim suffices for many pinholes, and seems to work smoother than aluminum cans. Thinner shim stock is fragile, and thicker shim stock requires more effort to make pinholes with thin edges.

I get 0.34mm and the only difference between mine and Jim's is "user constant" of 1.5 instead of 1.4. I tend to lean toward smaller with paper because blue and uv is usually so strong outdoors. The difference between .31 and .34 might be hard to detect.

Jim, I was not very clear. What I mean is that I've seen an odd almost split-toned effect on some long exposures. I was speculating that it's due to the mixture of different emulsions on VC paper. It's a puzzle: I also have some long exposures that don't show it. By long, I mean 10 to 20 minutes.
This is the most extreme example I've seen: http://www.flickr.com/photos/neds/8430730455